Topical sunscreen compositions are commonly used during outdoor work or leisure as a means for providing protection of exposed skin against acute and chronic adverse effects of solar radiation such as, for example, sunburn, cancer and photo-aging. Many effective sunscreen preparations are sold commercially or are described in cosmetic or pharmaceutical literature. In general, sunscreen preparations are formulated as creams, lotions or oils containing as the active agent an ultra violet radiation absorbing chemical compound. The sunscreen functions by blocking passage of ultra violet radiation thereby preventing its penetration into the skin.
According to Zecchino et al. (U.S. Pat. No. 5,008,100), sunscreen agents may be characterized in the order of decreasing effectiveness as either highly chromophoric (monomeric organic compounds and inorganic compounds such as titanium dioxide) and minimally chromophoric (polymeric organic solids).
Organic sunscreens are classified into UV-A filters, UV-B filters or broad spectrum filters (UV-A and UV-B functionality in a single molecule) depending on the type of radiation they absorb. UV-A sunscreens absorb radiation in the 320 to 400 nm regions of the ultra violet spectrum and UV-B sunscreens absorb radiation in the 290 to 320 nm regions of the ultra violet spectrum.
Broad band sunscreens (UV-A and UV-B functionality) absorb radiation in the 290 to 400 nm region of the ultra violet spectrum and have two maximums, one in the UV-B region and the other in the UV-A region.
Representative references related to UV sunscreens are:
U.S. Pat. No. 3,278,448, which discloses cinnamic acid derivatives such as 4-hydroxy, 3–5-ditertbutyl-alphacarbethoxy-cinnamic acid ether ester in column 2, line 20;
U.S. Pat. No. 3,538,226, which describes cinnamic acid alkyl ester derivatives at column 1, lines 15–31 and column 2, lines 1–12 and column 3, lines 30–55 and 60;
U.S. Pat. No. 5,175,340, which describes cinnamic acid alkyl esters having hydroxy radicals and alkoxy radicals on the phenyl ring, and
U.S. Pat. No. 5,830,441, which describes UV absorbents containing a cyano or cinnamyl moiety by the generic formula at col. 2, lines 1–21.
Other references which disclose cinnamide compounds include U.S. Pat. Nos. 5,601,811, 4,335,054, 5,124,354, 5,294,643 and 5,514,711.
Unfortunately, some of the highly chromophoric monomeric organic compounds employed in sunscreen compositions are not photostable and the protection from sun damage is lost. For example, Avobenzone, a UV-A sunscreen, is generally photo-unstable. Furthermore, photo-instability of Avobenzone increases significantly when combined with Octyl methoxycinnamate (UV-B organic sunscreen). Inmost studies, Octyl methoxycinnamte (OMC) has been regarded as relatively photostable. The absorption maxima of Avobenzone (˜360 nm) and OMC (˜310 nm) do not overlap sufficiently to allow directly excited singlet—singlet energy transfer to occur. However, transfer from one excited triplet-state to another is possible provided the energy levels are suitable.
The triplet-state of OMC has been shown to quench the triplet-states of 8-methoxy psoralen and 5-methoxy psoralen, subsequently undergoing E/Z isomerization (Morliere P., O. Avice, T. S. Melo, L. Dubertret, M. Giraud and R. Santus, A study of the photochemical properties of some cinnamte sunscreens by steady state and flash photolysis. Photochem. Photobiol, 36, 395–399 (1982); Morliere P., G. Huppe, D. Averbeck, A. R. Young, R. Santus, and L. Dubertret, In-vitro photostability and photosensitizing properties of bergamot oil. Effects of a cinnamte sunscreen. J. Photochem. Photobiol. B, Biol, 7, 199–208 (1990)). This is clearly evident that the triplet state of OMC is accessible to triplet energy transfer and that subsequent chemistry occurs.
The triplet-state of Avobenzone is surprisingly accessible and allowing energy transfer to other nearby molecules having triplet states with suitable overlap; the presence of OMC provides a suitable target acceptor. OMC undergoes E/Z photoisomerization upon accepting triplet state energy transfer and subsequent photolysis of OMC. It would appear that photo-instability of Avobenzone may cause the photolysis of OMC when they are combined together. This limits obtaining a broad-spectrum (UV-A and UV-B) sunscreen composition combining Avobenzone with Octyl methoxycinnamte.
In addition to lack of photostability of many organic sunscreens, they do not possess an antioxidant property, which protects skin or hair.
The ideal sunscreen formulation/composition should be nontoxic and non-irritating to the skin tissue and be capable of convenient application in a uniform continuous film. The product should be chemically and physically stable so as to provide an acceptable shelf life upon storage. It is particularly desirable that the preparation should retain its protective effect over a prolonged period after application. Thus, the active agent when present on the skin must be resistant to chemical and/or photo degradation.
Techniques for stabilizing UV absorbent compositions are known. Representative disclosures in this area include U.S. Pat. Nos. 5,567,418; 5,538,716; 5,951,968 and 5,670,140.
Antioxidants are believed to function by providing protection from free-radical damage. To be an effective quencher, it is believed the antioxidant must be present in an adequate concentration at the site of free radical generation. Since antioxidants are used in low concentrations and are a separate ingredient, they may not be available at the site of generation, thereby reducing the desired level of skin protection. Based on these drawbacks, it is desirable to provide a photostable sunscreen composition wherein a photostabilizer having an antioxidant and sunscreen functionality in a single molecule is present to enhance the effectiveness of the protection of skin against sun damage.